The present invention relates to a method for controlling an applied force, which is applied by a movable object (for example, a pair of welding tips of a welding gun for a spot welding) whose driving is controlled by a servo motor, to another object, and an apparatus therewith.
Generally, an applied force that is applied by a movable object whose driving is controlled by a servo motor to another object is controlled so that torque more than preset torque may not be generated from the servo motor, by applying a torque limit to an output of a speed control loop, that is, a torque command value.
For example, in the case of a welding gun for spot welding that is driven by a servo motor, work is pinched with welding tips of an electrode section of the welding gun, is pressed as a predetermined pressure generated by the output torque of the servo motor, and is welded by applying electricity between the welding tips.
In this welding gun, a move command is given to the servo motor driving the welding gun, the command which makes the servo motor advance further in a pressing direction over pressing points. Then, even if the electrode tips come in contact with work and are stopped, a moving length by the move command remains. Therefore, the servo motor attempts to make the electrode tips advance, and hence, the torque command value outputted from the speed loop increases to attempt in an make the electrode tips advance further. Then, by applying a torque limitation to this torque command value, the work is pressed with this constant output being torque limited.
The above method has a problem in that the vibration of the applied force arises because an end of the welding gun becomes a spring. Another problem is that cycle time is delayed because it takes time for the pressure to become a preset applied force since it takes time for the torque command value for the servo motor to reach the torque limitation value. Furthermore, if the welding tips of the electrode section of the welding gun, that are driven by the servo motor move an, acceleration torque for this movement is lost from the output torque limited, and hence the applied force is lacking.
An object of the present invention is to provide a method for controlling force application with a servo motor for not only preventing the applied force from vibrating but also preventing the applied force from delaying, and for preventing the applied force from lacking, and an apparatus therewith.
In order to achieve the above object, a method for controlling force application with a servo motor, in which an applied force of a movable object driven by a servo motor to another object is controlled, according to the present invention includes a step of forming a force control loop with an applied force commanded to the servo motor and the applied force estimated by an observer; and a step of performing feedback control of the applied force.
In addition, preferably, the method further includes a step of detecting the speed of the movable object to perform the feedback control of speed, too.
In addition, an apparatus for controlling force application with a servo motor, according to the present invention, controls an applied force of a movable object, driven by the servo motor, to another object, and includes an observer estimating the applied force; and force control loop execution means for performing the force feedback control with an applied force commanded to the servo motor and the applied force estimated by the observer.
In addition, preferably, the apparatus further includes speed detection means detecting the speed of the movable object; and speed feedback control means performing the speed feedback control.
Since the present invention performs feedback control of the applied force by the movable object (welding tips) driven by the servo motor, the present invention can generate the applied force, which is targeted, with good response. Furthermore, the present invention can accelerate the cycle time of work. Moreover, the present invention can adjust the applied force lest vibration should arise in the applied force, by adjusting the gain of the force feedback control and the gain of the speed loop provided in the force loop. Hence, the present invention can constantly generate the applied force, which is targeted, without the vibration arising during force application.